CHEM 2320 Lecture 9: 01.29.17

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01.27 .2017
Lecture #9
proton NMR Spectroscopy allows at to detect H's and The Cspectroscopy helps Us detect BC
.
How Manu signals we should expect depends on the symmetry of the molecule .
Diamagnetic Anisotropy
Having differences in hybridization can affect the
electro
negativity of that carbon .
sp2
SP >SPZ > sp3
t
SPZ moved it down to 7.3 ppm , whereas F only moved
It down to 4.5 . Nevertheless ,SPZ isn't more
electronegative
than Fluorine .
SP
¢
-???
These trends do not follow electronegative 'ty .
Diamagnetic Anisotropy helps explain these
Observations .
Anisotropy :different magnetic fields in different locations .
arises from the movement of it electrons .
Get The cycling of electrons around
the ring a magnetic
#
.It generates
field .
Owftahlidbnoed
Look at direction of the magnetic
field lines in the area where the
protons are residing .
larger aE→
For benzene ,the magnetic field is in an area where the protons Are aligned With Bo .This increase Bettective .
moving it to ahigher frequency (more downfield ).
Benzene Is normally in the order of 7~8 ppm due to diamagnetic anisotropy .
Here have asingle abond .
Have both diamagnetic anisotropy and
inductive effect Taking place simultaneously
G9.0 -100PM
in here .
a
7 PDM
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2Ibonds that are 9o° degrees so they have electrons
all the way around it .
OIn this case , the
field lines are opposed
to the magnetic
field . Bo .
Will therefore
Gcause shielding
/
Chemical Shift is Affected by Substitution
Tiedt
For aproton that's been affected by any of The things that we've seen already ,the more protons are
attached to this carbon ,the further it goer downfield .
Even it it were another atom ,like oxygen ,in the middle ,the same trend would occur .
chemical shift summary
H
at denudes are perk
Carboxylic acids A
protons on
double bonds .
Sharp
signal
broad
signal protons on cbearing
electronegative element
depends on EN next to Ti
of Z.bonds .CHCHZ CHZ
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